1. Field of the Invention
The present invention relates to a technical field of a carriage servo apparatus, an information reproduction apparatus and a carriage servo control method. More particularly, the present invention relates to a technical field of a carriage servo apparatus for moving a support member (hereinafter referred to as the “carriage”) that moves in a direction parallel to an information recording face while supporting a pickup for optically recording or reproducing information relevant to the information recording face; as well as an information reproduction apparatus having such a carriage servo apparatus and a carriage servo control method.
2. Description of the Related Art
In general, in the case where information is optically recorded or reproduced relevant to an information recording medium such as optical disc, so called servo control is required to ensure that a focusing position of light beams such as the recording or reproduction laser light is precisely coincident with a position of an information track on an information recording face on which the information in the information recording medium is to be recorded or reproduced.
At this time, the servo controls include so called focus servo control for servo controlling coincidence between the focusing position in a direction vertical to the information recording face and an information track position and so called tracking servo control and carriage servo control for servo controlling coincidence between the focusing position in a direction parallel to the information recording face and vertical to the above information track and the information track position.
Here, the carriage servo control denotes servo control executed when a carriage being a support member for supporting a pickup that ejects the above light beams and receives reflection light from the information recording face, and moving in a direction vertical to the above information track (its radial direction in the case of an optical disc) is moved in a direction vertical to the information track.
In addition, with respect to a relationship between the tracking servo control and the carriage servo control, the tracking servo control moves an objective lens in a pickup for focusing the light beams on an information recording face in a direction vertical to an information track, thereby making fine adjustment for a light focusing position. On the other hand, even when an objective lens reaches a movable limit position on a preset design in its movement reaches, in the case where the focusing position is displaced from the information track position, the carriage servo control moves a pickup itself in a direction vertical to the information track, thereby eliminating the displacement between the focusing position in a direction parallel to the information recording face and the information track position.
At this time, with respect to the actual tracking servo control and carriage servo control, specifically based on reflection light of the above light beams, a so called tracking error signal indicating a displacement in a direction vertical to an information track (direction parallel to an information recording face) between the light focusing position and the information track position is first generated by a technique such as 3-beam technique, a 1-beam phase differential technique or heterodyne technique and the like. Then, the tracking servo control and carriage servo control are performed so that a level of this tracking error signal becomes zero.
Here, in a low frequency bandwidth in the tracking error signal (in a bandwidth for a low frequency equal to or smaller than a frequency that corresponds to an eccentricity component in rotation of an optical disc), as shown in FIG. 10(a), there are included an alternating current component that corresponds to the above eccentricity component or the like in the case of the optical disc, on the one hand, and a direct current component generated due to the fact that the information track is formed spirally in the optical disc (that is, due to the fact that the focusing position should be moved in an outside (or inside) direction continuously and gradually together with rotation of the optical disc), on the other hand. At this time, the above carriage servo control is required to eliminate displacement generated beyond the above movable limit in the objective lens, of the positional displacements indicated by the direct current component.
In the actual carriage servo control, this tracking error signal is converted into a waveform suitable to the driving of a drive device such as carriage motor for moving a carriage as shown at the upper part of FIG. 10(b). Then, only the tracking error signal beyond a preset threshold VZ that corresponds to the above movable limit is defined as a drive signal (refer to the lower part of FIG. 10(b)) to be applied to the drive device, thereby moving a carriage.
When the above movable limit of the objective lens is exceeded, the carriage servo control is executed. Thus, the drive signal is intermittently applied as shown at the lower part of FIG. 10(b). Further, an original tracking error signal produced when the carriage servo control is executed is discontinuously changed along with carriage movement, as shown in FIG. 10(a).
In addition, only a tracking error signal having a level of the threshold VZ or more after conversion of waveform is applied to the drive device. This is because, in an arrangement for applying all the tracking error signal to a drive device without providing a threshold, application of the tracking error that corresponds to a range in which a carriage cannot be moved becomes wasteful processing, and wasteful power consumption caused by such application occurs.
Further, in the case where the waveform of the above drive signal supplied to the above drive device for moving a carriage (generally, so called DC (direct current) motor is employed.) is formed in a pulse shape as shown in FIG. 10(b), that torque is generally determined in proportion to a current supplied to the drive device. In diagram of this fact, there is an advantage that a pulse shaped drive signal rising steeply like a so called step function rather than that having its waveform gradually rising like a so called ramp function can generate a large current value, and thus, a startup voltage of the drive device can be reduced.
At this time, conventionally, in order to simplify an arrangement, this threshold VZ is kept identical and unchanged relevant to the same type of servo control device for performing the carriage servo control.
However, in the actual carriage servo control, for example, due to mechanical factors such as stiffness of mating of carriage movement gears or electrical factors such as voltage shift in a drive control device included in a driving apparatus, there is included a significantly large deviation (for example, deviation in individual information reproduction apparatus for performing carriage servo control) in a voltage (hereinafter referred to as the “carriage startup voltage”) required to be applied to a driving apparatus when a carriage starts to move.
At this time, in the carriage servo control including such deviation in carriage startup voltage, in the case where the voltage VZ is constant as described above, for example, even if a carriage startup voltage on design is applied, a carriage does not move. As a result, there has been a problem with unstable operation that a large carriage startup voltage is applied, whereby the carriage moves excessively; even if the same carriage startup voltage is applied, the carriage moves more excessively than a designed value; and excessive power consumption occurs.
This problem leads to a problem that precise carriage servo control is executed as a result of unstable carriage movement, and information cannot be recorded or reproduced precisely.